1. Field of the Invention
This invention relates optical phase conjugation and, more specifically, to a phase conjugate mirror (PCM) using a gasdynamic gas flow to provide phase correction in a distorted wavefront of a coherent light, i.e., a laser beam.
2. Brief Description of the Prior Art
Phase correction methods are required to improve the laser beam wavefront (sometimes referred to as phasefront), for example, in applications that deliver laser power to a remote target, in atmospheric propagation and in producing a high-brightness and energy laser beam by combining many separate large-aperture, non-uniform intensity laser beams. The improvement in beam wavefront results in higher brightness (power radiated by a point source into unit solid angle) output so that more power is delivered to the target. This brightness is highest ideally for a diffraction-limited beam which has a uniform-spatial-coherent wavefront, and the lowest beam divergence, resulting in minimal beam spreading due to diffraction, in propagation over large distances. The beam's wavefront is usually degraded if the high-quality laser beam propagates through a linear or nonlinear phase aberrating medium (e.g., earth's atmosphere, poor optical quality components, etc.). Distortion correction can occur if it is possible to send a laser beam with a reversed (i.e., negative) phasefront in a direction opposite to that of the incident laser beam through the aberrating medium. This reversal of the phasefront is called phase conjugation (PC), because in the theory of light, the electric field of light is mathematically conjugated to achieve phase reversal.
There are two principal ways to effect phase conjugation in a laser beam: adaptive optics and nonlinear optics. In the adaptive optics approach, a coherent light beam is sent through the aberrating medium, its phasefront is measured, and then this phase information is impressed on the outgoing laser beam via an electro-optical or mechanical deformable mirror. This linear optical approach assumes the aberrations are varying relatively slowly during this phase information transfer process, so that the return phase conjugated beam is wavefront-corrected after propagating the second time through the aberrating medium. Note that there are other possible optical schemes for faster response, high resolution adaptive wavefront estimation and compensation (for example, see the "Adaptive Gas Lens" disclosed by P. J. Goede in U.S. Pat. No. 4,721,370). The other approach is to use a nonlinear optical phase conjugation method to produce the phase conjugated beam. After passing through the aberrating medium, the phase-aberrated light beam impinges on a phase-conjugate mirror (PCM) which reverses the phasefront and reflects the incoming beam to produce the phase conjugate output beam. This PC beam passes through the aberrating medium again, to produce the reverse wavefront, which is, ideally, identical to the incident, undistorted phasefront at that position. This also assumes that the phase distortions do not change significantly during the round-trip time the light beam traverses the aberrating medium. Since PC occurs quite rapidly (an important exception is self-phase-conjugating photorefractive mirrors which respond slowly--on the order of seconds; these PCMs are used with low power continuous-wave lasers) in most instances, this approach is used to reduce beam distortions inside a laser system.
Two approaches are generally used to implement a nonlinear optical PCM: stimulated scattering (SS) and four-wave mixing. In the SS approach, the beam is usually focussed by a lens into an SS cell where exceeding the intensity threshold for stimulated scattering (usually a stimulated Brillouin scattering (SBS) process) results in a phase-conjugated wavefront which then retraces the path through the aberrating medium. The SS approach results in a simple setup for the PCM since the backward-scattered PC process can result from noise amplified by the SS, and therefore requires no extra beams or pump sources to achieve PC. For an SBS process, the backward-travelling PC wavefront is frequency shifted in about 1 part in 1.times.10.sup.4 to 1.times.10.sup.5 relative to the input beam frequency and is comparable in intensity and beam quality to the incoming laser beam. The SS approach has the disadvantage of requiring an intensity threshold to activate the PCM. This SS intensity threshold can be reduced by increasing the exponential gain factor for SS by increasing the interaction length through which the incident beam travels. The interaction length can be increased by using an SS lightguide cell. The theory to predict SS-PC in an optical waveguide (i.e., lightguide) was first described in "Connection Between the Wavefronts of the Reflected and Exciting Light in Stimulated Mandel'shtam-Brillouin Scattering," by B. Ya. Zel'dovich et al., Vol. 15, pp. 160-164 (1972) (English translation: JETP Letters, Vol. 15, pp. 109-113 (1972)). Since that time, there has been a growing number of experimental and theoretical work performed on PC lightguides.
In four-wave mixing, three input laser beams are combined in a nonlinear medium to produce the phase conjugate beam. Note that one of the three input beams is phase conjugated. The four-wave mixing approach does not require an intensity threshold to be reached to produce the PCM reflection of the input beam, however the requirement for three input beams results in higher system complexity which can be disadvantageous in some applications. Moreover, for high power laser beams, the pump beam quality may not be sufficient for effective phase conjugation. However, for lower power applications, it is possible for power to be transferred from the input pump beams to the phase conjugate beam, producing gain in the intensity of the phase conjugated replica relative to the input beam. For example, Scott and Hazell recently reported PCM reflectivities greater than 1.times.10.sup.4 in a stimulated Brillouin-enhanced four-wave mixing setup ("High-Efficiency Scattering in Transient Brillouin-Enhanced Four-wave Mixing," A. M. Scott and M. S. Hazell, IEEE Journal of Quantum Electronics, Vol. QE-22, pp. 1248-1257 (1986)).
The development of high-energy lasers for fusion applications has led to a number of experimental and theoretical studies of suitable nonlinear processes for shortening (i.e., pulse compression) the laser pulsewidth. These studies have shown that gas-phase PC is attractive for high-energy laser applications. Furthermore, gases are less likely to suffer from other competing nonlinear processes (such as self-focussing in liquids), are more resistant to optical damage since gases can withstand very high light intensities, &gt;2.times.10.sup.9 watts per centimeter squared ("Raman Pulse Compression of Excimer Lasers for Application to Laser Fusion," J. R. Murray et al. IEEE Journal of Quantum Electronics, Vol. QE-15, pp. 342-368 (1979)), and high pressures can be achieved to increase the gain for SS (notably SBS, for PC). However, the high gas pressures, ranging from 10-100 atmospheres, needed for efficient PC, places a constraint on the choice of gas to be used in a particular application. This is due to absorption of laser light, especially in the ultraviolet and infrared spectral region, where most molecular gases absorb appreciably at high pressures.
The initial experiments on gas optics were performed in the 1960s to explore the use of long, narrow gas cells as periodic lens waveguides for communication applications. A vast number of theoretical and experimental studies of gasdynamic supersonic gas flows have been performed for more than the past 35 years, in particular, the application of supersonic jets to produce intense atomic and molecular beams with well-defined density flow fields. There have been experimental results demonstrating the application of a gasdynamic supersonic jet to produce the gasphase, nonlinear medium for the production of vacuum ultraviolet (VUV) coherent light using nonlinear optical harmonic generation. Possible phase conjugation effects were not reported ("Generation of 35.5-nm Coherent Radiation," J. Bokor et al., Optics Letters, Vol. 8, pp. 217-219, (1983); "Third Harmonic Generation in a Pulsed Supersonic Jet of Helium," A. H. Kung, Optics Letters, Vol. 8, pp. 24-26 (1983); A. H. Kung U.S. Pat. No. 4,577,122; and "Pulsed Free Jets: Novel Nonlinear Media for Generation of Vacuum Ultraviolet and Extreme Ultraviolet Radiation," C. T. Rettner et al., Journal of Physical Chemistry, Vol. 88, pp. 4459-4465 (1984)). Also, experimental results have been reported on a high optical quality lightguide produced from a hyperbolic-shaped gasdynamic jet, and good agreement was found between the theoretically-calculated and experimentally-observed shapes of the lightguiding regions ("Gasdynamic Lightguide with High Optical Quality," D. W. Bogdanoff, Applied Optics, Vol. 24, pp. 2005-2013 (1985)).